1. Field of the Invention
This invention relates to semiconductor fabrication, and more particularly to a method for fabricating a thin-film transistor (TFT).
2. Description of Related Art
Recently, a TFT has wide applications in a liquid crystal display (LCD) and other related products. The TFT typically includes two types: one is an amorphous TFT, and the other one is a polysilicon TFT. Generally, the amorphous-silicon TFT has lower leakage current but its field effect mobility is too low, causing its activating current to be low. The is the main problem of the amorphous-silicon TFT. The polysilicon TFT has higher field effect mobility with a larger activating current but its leakage current cannot be effectively reduced. The leakage current causes an increase of a switching current (I.sub.on /I.sub.off) The polysilicon TFT therefore cannot be applied in a large area LCD. It becomes an important issue for a TFT to increase the field effect mobility and reduce the switching current.
In the conventional technology, a lightly doped drain (LDD) structure can reduce leakage current but it needs a more complicate fabrication process. K. Y. Choi et al. And J. H. Jeon et al. proposed to use amorphous silicon (a-Si) as an offset structure in 1997 at SSDM. In this proposal, the leakage current can also be effectively reduced but its structure is still complicate.